Engineering S-scheme Ag2CO3/g-c3N4 heterojunctions sonochemically to eradicate Rhodamine B dye under solar irradiation

The use of natural solar radiation is a low-cost significant technology for water pollution remediation and production of clean energy. In this work, S-scheme Ag2CO3/g-C3N4 heterojunctions were engineered for carefully eradicating Rhodamine B dye under natural sunlight irradiation. Solid thermal decomposition reactions generate g-C3N4 sheets by annealing urea at 520 °C. Ag2CO3 nanoparticles are directed and localized sonochemically to the active centers of g-C3N4 sheets. The physicochemical properties of the solid specimen were determined by PL, DRS, XRD, HRTEM, mapping, EDX, N2-adsorption–desorption isotherm and XPS analyses. As elucidated by HRTEM, PL and DRS analyses, 5 wt% of spherical Ag2CO3 nanoparticles deposited on the g-C3N4 sheet surface and nearly equidistant from each other elevate the electron–hole separation efficiency and broaden the absorption capacity of photocatalysts. Rhodamine B dye was degraded at a rate of 0.0141 min−1 by heterojunctions containing 5 wt% Ag2CO3 and 95 wt% g-C3N4, which is three-fold higher than that on pristine g-C3N4 nanosheets. Free radical scrubber experiments revealed the contribution of charge carriers and reactive oxygen species to the decomposition of RhB dye with a preferential role of positive holes and superoxide species. PL measurements of terephthalic acid and scrubber trapping experiments provide confirmatory evidence for charge diffusion via the S-scheme mechanism that accounts for the production of electron–hole pairs with strong redox power. This novel research work is contributory to manipulate the S-scheme heterojunction for efficient and low-cost wastewater treatment under natural solar irradiation.


Introduction
Water pollution is an environmental crisis that urgently needs to be addressed to meet the increasing demand for fresh water. Organic dyes are the major toxic pollutants discharged from different industries causing serious problems to human health and aquatic life. [1][2][3][4] Adsorption, reverse osmosis, electrochemical process, coagulation, enzymatic treatment and ionexchange processes are very expensive, complicated techniques that produce a second generation of pollutant materials. [5][6][7][8][9][10] Photocatalysis on the surface of semiconductors is a green technology for the destruction of organic wastes into eco-friendly species. The photocatalytic reaction requires two factors to achieve optimization conditions. The rst one is a low-cost and broadband solar radiation source that is sufficient to decompose millions of molecules of pollutants. The second requirement is an appropriate semiconductor with tunable band gap energy and extremely high surface area that generates huge amounts of reactive oxygen species, which can destruct toxic pollutants in a short time under light irradiation. Natural sunlight in our country is very abundant in all seasons, particularly in summer days and acts as a low-cost radiation source. The natural sunlight contains a minor amount of UV radiation (5%), visible radiation (43%) and near infra-red (NIR) radiation (50%). Most of the previous research studies have focused on investigating the photocatalytic process under UV and visible light irradiation. In recent years, NIR is very effective in decomposing organic pollutants via photocatalytic and thermal routes. An appropriate semiconductor from the perspective of thermodynamics and kinetics must have the advantages of low cost, environmentally benign nature, high separation rate and fast transfer of charge carriers. Efficient semiconductors with wide bandgap structures as TiO 2 , SnO 2 , ZnO and CeO 2 show high rate of organic pollutant decomposition [11][12][13][14][15][16] under UV irradiation that constitutes only 5% of the natural solar radiation. Narrow band gap energy semiconductors absorb visible light radiation; however, the ultra-fast electron-hole pair coulombic attraction decreased the life time required to generate reactive oxygen species. [17][18][19][20] In conclusion, a single semiconductor cannot achieve the optimization properties for decomposing the organic pollutants. To overcome the limitations of single photocatalysts, heterojunctions composed of wide band gap energy with high efficiency for charge carrier separation and narrow band gap energy that harvests solar radiation are more favored. Recently, graphitic carbon nitride (g-C 3 N 4 ), has gained great interest in photocatalytic reactions due to its good stability in basic and acidic media, low synthesis costs, non-toxicity without side effects, sufficient biocompatibility, thermal stability, and a narrow band gab energy of about 2.7 eV and promote the photocatalytic activity under visible light. [21][22][23][24][25] Low specic surface area (∼50 m 2 g −1 ) and ultra-fast attraction of electronhole pairs inhibit the prolonged time of charge carriers in the photocatalytic process. The low valence band potential (E VB = +1.5 eV) of g-C 3 N 4 fails to generate hydroxyl radicals ($OH), which are essential species for the destruction of organic pollutants. The hybridization of wide and narrow band gap energy semiconductors is a promising issue for successfully generating heterojunctions with appropriate solar radiation absorbability and high separation and transportation efficiency of electron-hole pairs. Silver-based photocatalysts such as AgCl, Ag 3 PO 4 , AgIO 4 , AgVO 3 and Ag 2 CO 3 exhibit strong photocatalytic reactivity in the decomposition of organic pollutants. [26][27][28][29][30][31] Much attention is drawn towards Ag 2 CO 3 nanoparticles due to their insolubility in water, low toxicity, adjustable bandgap structures and simple synthesis routes. [32][33][34][35][36][37][38][39][40][41] The successful construction of heterojunctions requires the appropriate adjustment of the band energy structure of the two semiconductors to generate charge carriers with an auspicious redox power. [42][43][44] Recent research studies have focused on coupling Ag 2 CO 3 with g-C 3 N 4 to generate a successful heterojunction for removing organic containments from wastewater. [45][46][47][48][49][50][51] Konglin et al. reported the successful photodegradation of Rhodamine B and methylene blue dyes over g-C 3 N 4 /Ag 2 CO 3 containing 3.5 wt% g-C 3 N 4 , which was ascribed to the predominant role of g-C 3 N 4 in limiting the electron-hole recombination. 45 Lei et al. synthesised g-C 3 N 4 /Ag 2 CO 3 containing 25 wt% g-C 3 N 4 via a sonochemical route for destructing RhB dye under a 300 W xenon lamp, and the experimental results manifested that g-C 3 N 4 increases the life time for charge carriers, which is responsible for the degradation process. 46 Shugang et al. prepared Ag 2 CO 3 /g-C 3 N 4 containing 40% Ag 2 CO 3 by a co-precipitation process for removing methyl orange and methylene dyes. The remarkable reactivity of the nanocomposite was ascribed to the inuence of Ag 2 CO 3 in enhancing the electron-hole transportation and separation. 48 Yun Feng et al. prepared Ag 2 CO 3 /g-C 3 N 4 containing 30% Ag 2 CO 3 by a precipitation method for the removal of methyl orange and methylene blue. The exceptional reactivity is attributed to the role of Ag 2 CO 3 in enhancing the efficiency of charge carrier separation. Previous research studies concentrated on doping silver carbonate as a major constituent with g-C 3 N 4 , which is considered a high-cost route for water treatment. In this work, we made an attempt to construct S-scheme Ag 2 CO 3 /g-C 3 N 4 with a minimum amount of silver carbonate. The previous research explores the construction of Ag 2 CO 3 /g-C 3 N 4 via type (II) heterojunctions and direct Z-scheme mechanism. The type (II) heterojunction fails from dynamic, thermodynamic and energetic points of view to explain the actual charge transportation. The electron transfer from high to low conduction band is accompanied by repulsion force between the existing and the transferring electrons. Concurrently, the electron diffusion from high to low energy level dissipates electrons with a strong reducing power. Direct Zscheme fails to explain the precise analysis of charge migration between the two semiconductors. The construction of Sscheme Ag 2 CO 3 /g-C 3 N 4 heterojunctions has not been explored in previous research studies. Hybridizing Ag 2 CO 3 with high positive oxidative potential and g-C 3 N 4 with high negative reductive potential generate successful S-scheme heterojunctions. The Fermi level and conduction band of g-C 3 N 4 is higher than those of Ag 2 CO 3 ; however, the work function of Ag 2 CO 3 is greater than that of g-C 3 N 4 . Upon light illumination, the Fermi levels of Ag 2 CO 3 and g-C 3 N 4 jump upward and downward until the two Fermi levels are contacted at the interface region. At this contact point, the electrons and holes with low redox potential are attracted toward each other, and vanished leaving a strong internal electric eld. Concurrently, the holes and electrons in the higher valence and conduction bands with a strong redox power are consumed in the photocatalytic process. The production of S-scheme systems is more favorably engineered via a solution-based approach to control and tune the particle structure and pore matrix. The sonochemical route is a professional solution process for the synthesis of Ag 2 CO 3 /g-C 3 N 4 heterojunctions with the homogeneous location of Ag 2 CO 3 on g-C 3 N 4 active centers. The assynthesized heterojunction was subjected to decomposition of Rhodamine B under solar irradiation to utilize UV, visible and near infra-red radiations. Exclusively, NIR light elevates the reaction temperature through the photothermal effects, which enhances the photocatalytic activity. In this work, we aimed to synthesise g-C 3 N 4 from microcrystalline urea via a thermal decomposition process. Ag 2 CO 3 nanoparticles were hybridized with g-C 3 N 4 sheets in an ultrasonic bath of 300 W intensity. The physicochemical properties of the solid specimens were characterized by XRD, FTIR, HRTEM, EDX, XPS, mapping, N 2 adsorption-desorption isotherm, DRS and PL analyses. The photocatalytic activity of the heterojunctions was explored by following the degradation of Rhodamine B dye under natural sunlight radiation of 500 W intensity. The key role of the oxygen radicals and charge carriers was elaborated by carrying out various trapping scavenger experiments and following the PL analysis of terephthalic acid as a probe radical material. On the basis of DRS, PL and trapping scavenger analyses, a proposed mechanism for the transportation of charge carriers between Ag 2 CO 3 and g-C 3 N 4 semiconductors in the circuit of the heterojunction is illustrated.
2.2. Preparation of g-C 3 N 4 nanosheets g-C 3 N 4 sheets were synthesized from urea aer purication via recrystallization by 200 mL of methanol (99%) with continuous stirring at 150°C. The solution was allowed to stand at room temperature to get rid of volatile methanol carefully. Then, 100 g of recrystallized urea was annealed in a reactor made of Fig. 1 Scheme for the synthesis of Ag 2 CO 3 /g-C 3 N 4 nanocomposites. aluminum metal at 520°C for 3 hours and the obtained solid was washed with distilled water and dried at 80°C. Finally, a yellow solid specimen was obtained and ground in a porcelain mortar to generate g-C 3 N 4 nanosheets.

Preparation of Ag 2 CO 3 nanoparticles
First, 6 g AgNO 3 dissolved in 100 mL distilled water was added to a solution containing 2 g Na 2 CO 3 dissolved in 100 mL distilled water with continuous stirring for 1 hour to ensure homogenous distribution. The solution was sonicated for 30 min in an ultrasonic bath at 300 W intensity followed by ltration, washing with distilled water and drying at 110°C. A dark yellow solid was obtained, ground and stored in a falcon tube. Fig. 1 illustrates the plausible scheme for the synthesis of Sscheme Ag 2 CO 3 /g-C 3 N 4 heterojunctions via a sonochemical route. Typically, a denite amount of silver carbonate nanoparticles dispersed in 20 mL of distilled water were sonicated with g-C 3 N 4 nanoparticles for 30 min by certain ratio to obtain (5, 10 and 20 w/w%) Ag 2 CO 3 /g-C 3 N 4 . Aer a while, each of the solution mixture was stirred for 1 h, ltered, washed with distilled water and dried at 80°C. The photocatalysts are denoted as g-C 3 N 4 , Ag 2 CO 3 , CNAg5, CNAg10 and CNAg20 for pristine g-C 3 N 4 , pristine Ag 2 CO 3 and the heterojunctions containing 5, 10 and 20 wt% Ag 2 CO 3 , respectively.

Material characterization
A PANalytical X'PERT MPD diffractometer with Cu [Ka 1 /Ka 2 ] radiation was employed to investigate the crystalline properties of the as-synthesized heterojunctions. FTIR spectroscopy revealed the functional groups of the nanocomposite samples. Adsorption isotherms of N 2 at 77 K precisely investigated the surface parameters and pore structure of the solid specimens. HRTEM (JEOL 6340) visualized the particle size distribution of Ag 2 CO 3 on the sheets of g-C 3 N 4 . A K-ALPHA (Themo Fisher Scientic, USA) instrument with monochromatic X-ray Al Ka radiation in the range of 10-1350 eV was used for XPS analysis. JASCO spectroscopy (V-570) was performed to analyze the diffuse spectrum of the photocatalyst and determine the band energy structure. A lumina uorescence spectrometer (Thermo Fisher Scientic) was used to analyze the efficiency of photogenerated electron-hole separation.

Photocatalytic degradation of RhB dye
Typically, 0.1 g of the photocatalyst is mixed with 100 mL of 10 mg L −1 RhB dye in a solar reactor. The suspension was continuously stirred for 1 hour in the darkness until adsorption/desorption equilibrium was reached. Aer a while, the mixture solution was exposed to natural sunlight (450 W) at about 3 p.m. (Aug. 2021) for two hours. Then, 4 mL of the mixture was collected at denite time intervals, and then the photocatalyst was separated by centrifugation at 4000 rpm for 10 min. The photocatalytic degradation progress was followed by tracing of the reduction of dye color intensity using a UVvisible spectrophotometer. The photocatalytic mechanism and nature of reactive species were investigated using free radical scrubbers such as AgNO 3 , isopropanol, ammonium oxalate and benzoquinone for detecting conduction band electrons, hydroxyl radicals, holes and superoxide radicals. The photoluminescence of terephthalic acid was investigated to monitor the production of hydroxyl radicals at an excitation wavelength of 325 nm.

Results and discussions
XRD spectrum explores the crystalline structure of pristine g-C 3 N 4 , Ag 2 CO 3 , CNAg5, CNAg10 and CNAg20 nanocomposites [ Fig. 2]. Prevailing diffraction peaks of g-C 3 N 4 were recorded at 13.2 and 27.59°, which were ascribed to the (100)  .3 and 6.5 nm for g-C 3 N 4 , Ag 2 CO 3 , CNAg5, CNAg10 and CNAg20 nanoparticles, respectively. The XRD pattern of the CNAg5 nanocomposite containing 5 wt% Ag 2 CO 3 resembled the diffraction pattern of g-C 3 N 4 with complete missing of the diffraction peaks assigned to Ag 2 CO 3 , revealing the homogeneous dispersion of Ag 2 CO 3 between g-C 3 N 4 sheets. The prevailing diffraction peaks assigned to monoclinic Ag 2 CO 3 were vividly recorded in the nanocomposites containing 10 and 20 wt% Ag 2 CO 3 . The chemical interaction between g-C 3 N 4 and Ag 2 CO 3 was investigated by the FTIR spectrum. In Fig. 3a, the spectrum of g-C 3 N 4 shows various peaks in the range of 1100-1700 cm −1 assigned to the stretching vibration of heterocyclic C-N and C]N bonds and Fig. 2 (a) XRD pattern of g-C 3 N 4 , Ag 2 CO 3 , CNAg5, CNAg10 and CNAg20.
the peak at 808 cm −1 is attributed to the triazine units, and the broad peak in the range of 3000-3500 corresponds to N-H and O-H bonds of physically adsorbed water. For the spectrum of Ag 2 CO 3, four peaks were found at 720, 800, 1320, and 1430 cm −1 , indicating the presence of the CO 3 2− group. N 2adsorption-desorption isotherms of pristine g-C 3 N 4 and CNAg5 were classied as type (II) with a closed hysteresis loop, which was ascribed to the non-porous structure [ Fig. 4]. The surface area of g-C 3 N 4 and CNAg5 is 55.5 and 44.3 m 2 g −1 according to the BET equation in its normal range of applicability. The existence of such open surface prohibits the photocatalytic reaction with unrestricted accommodation of pollutant molecules. However, porous systems restrict the diffusion and transportation of RhB molecules due to the pore constrictions. The HRTEM image of the CNAg5 heterojunction is represented in Fig. 5, which records the generation of g-C 3 N 4 sheets with laminar structure containing various wrinkle points. On careful exploration of HRTEM images at different magnications, one can notice the successful deposition of spherical Ag 2 CO 3 nanoparticles on the active sites of g-C 3 N 4 sheets at equidistant positions in the homogeneous arrangement, which reveals the strong chemical interaction between Ag 2 CO 3 nanoparticles and g-C 3 N 4 sheets. The particle size distribution was elucidated by constructing histograms that reveal that major nanoparticles exhibit a size varying between 15 and 20 nm. HRTEM records the existence of lattice fringes of spacing 0.315 nm, which were ascribed to the (100) plane of Ag 2 CO 3 . SAED analysis manifests the existence of different rings ascribed to the crystalline planes of g-C 3 N 4 and Ag 2 CO 3 nanoparticles. Mapping and EDX elemental analysis results are illustrated in Fig. 6, revealing the uniform distribution of C, N, Ag and O in the CNAg5 heterojunction. The production of porous graphitic carbon nitride sheets with spongy structure was clearly observed with the homogeneous distribution of Ag 2 CO 3 on the localized active sites on g-C 3 N 4 sheets. The oxidation state and elemental distribution of the   The color of the nanocomposite became darker upon the incorporation of various contents of silver carbonate, revealing the shiing in the solid specimen's response to the visible light absorbability. Fig. 8 illustrates the DRS analysis of pristine g-C 3 N 4 , Ag 2 CO 3 and Ag 2 CO 3 /g-C 3 N 4 heterojunctions. The band gap energy was calculated according to the Tauc equation to determine the type of the electronic transition. The shiing in light absorbability of g-C 3 N 4 from 420 to 470 nm is sufficient to enhance the photocatalytic reactivity of the solid specimen in the visible light region. The valence and conduction band potentials were calculated on the basis of band gap energy and the electronegativity of g-C 3 N 4 and Ag 2 CO 3 . The valence and conduction band potentials of g-C 3 N 4 are +1.57 and −1.13 eV. However, the valence and conduction band potentials of Ag 2 CO 3 are +2.55 and +0.45 eV. The electron-hole separation efficiency for single-phase g-C 3 N 4 and the as-synthesized heterojunctions was explored by constructing the PL spectrum of solid specimens [ Fig. 9]. A fantastic emission signal at an intensity of 445 nm belongs to the electron-hole coulombic attraction recorded in the PL spectrum. About 40, 45, 68 and 73% reduction in the PL peak intensity was detected with the introduction of 5, 10, 15 and 20 wt% Ag 2 CO 3 on the g-C 3 N 4 surface. These results clearly indicated the signicant role of Ag 2 CO 3 in decreasing the electron-hole coulombic attraction force as well as promoting strong interactions between Ag 2 CO 3 and g-C 3 N 4 .

Photocatalytic degradation of Rhodamine B dye
The ubiquitous existence of dyes and antibiotics in wastewater is an alarming environmental issue that affects human health and aquatic life. Among the industrial dyes, Rhodamine B is one of the most employed dyes in industries. Rhodamine B dye (RhB) is toxic, carcinogenic, and non-biodegradable that poses a serious threat to human health. Various routes were developed for the safe destruction of RhB dye into eco-friendly species without producing a secondary generation of pollutants. Photocatalytic destruction of RhB under natural solar irradiation into CO 2 , water and small molecules is an auspicious route for expelling RhB dye from wastewater. The efficiency of the as-synthesized heterojunctions in degrading RhB was tested and compared under natural sunlight irradiation [ Fig. 10] to investigate the inuence of change in silver carbonate concentration on the efficiency of g-C 3 N 4 sheets. The decomposition of Rhodamine B dye under natural sunlight irradiation in the absence of photocatalysts is negligible due to the strong chemical stability of the dye solution. Pristine g-C 3 N 4 sheets exhibit poor efficiency in degrading RhB dye due to the ultra-fast recombination rate of electron-hole pairs and mild absorption responsibility under solar irradiation. On the contrary, pristine Ag 2 CO 3 shows high efficiency in degrading RhB dye, which results from the matching in band gap energy with sunlight absorbability. However, the high cost of the silver precursor hampers the industrialization of Ag 2 CO 3 for wastewater treatment. The incorporation of small amounts of Ag 2 CO 3 on the surface of low-cost semiconductors is a promising route for the manipulation of efficient heterojunctions with a strong redox power. Compared with pristine g-C 3 N 4 , the heterojunctions containing 5, 10, and 20 wt% recorded a fast rate of dye decomposition that reached 95% of the dye initial concentration on the surface of heterojunctions containing 5 wt% Ag 2 CO 3 [ Fig. 11a]. The decomposition of RhB dye passes through several intermediates generated by the expelling of the ethyl groups one by one. This process is trusted by observing various absorption peaks at different wavelengths arranged in descending order. The process of de-ethylation of the fully N,N,N − ,N − -tetra-ethylated Rhodamine molecule generates different intermediates as N,N,N − -tri-ethylated Rhodamine, N,N − -di-ethylated Rhodamine, and N-ethylated Rhodamine at 540, 522 and 502 nm, respectively. The rate of pseudo rst order of the dye degradation is 0.0012, 0.0434, 0.0141, 0.0123 and 0.0098 over g-C 3 N 4 , Ag 2 CO 3 , CNAg5, CNAg10 and CNAg20, respectively [ Fig. 11b]. The kinetic results indicated that the rate of dye degradation over CNAg5 is fourfold higher than that on pristine g-C 3 N 4 . Scrubber trapping experiments using benzoquinone, isopropanol, ammonium oxalate and silver nitrate were carried out to explore the role of superoxide and hydroxyl radicals besides the positive hole and electron conduction band on the destruction of RhB dye. The experimental result recorded a preferential retardation in RhB dye mineralization in the presence of benzoquinone, isopropanol and ammonium oxalate, which directed the attention toward the positive role of reactive oxygen species and positive hole in the photocatalytic reaction [ Fig. 11c]. The production of OHc groups under solar irradiation on the CNAg5 surface was recorded by following the intensity of PL emission signals of hydroxyl terephthalic acid at 424 nm [ Fig. 11d]. The peak intensity elevates with the increase in the irradiation time, which is taken as evidence for increase in the production of OHc species. The exceptional photocatalytic reactivity of the assynthesized samples was attributed to the construction of Sscheme Ag 2 CO 3 /g-C 3 N 4 heterojunctions. This novel heterojunction harvests the full broad spectrum and enhances the efficiency of electron-hole pair separation and transportation under solar irradiation. The S-scheme heterojunction is composed of a Ag 2 CO 3 oxidative photocatalyst and a g-C 3 N 4 reductive photocatalyst. Upon light illumination, electrons are  transferred from g-C 3 N 4 with a greater Fermi level and more negative conduction band potential to Ag 2 CO 3 with a low Fermi level [ Fig. 12]. Concurrently, the Fermi levels of Ag 2 CO 3 and g-C 3 N 4 photocatalysts jump upward and downward until the two Fermi levels are equalized. The fruitless holes and electrons of g-C 3 N 4 and Ag 2 CO 3 respectively are attracted to each other by coulombic attraction force and vanished. On the contrary, the hot holes of Ag 2 CO 3 and electrons of g-C 3 N 4 with a higher redox potential were consumed in the photocatalytic process. This mechanism is manifested from the scrubber experiments and the emission PL analysis of hydroxyterephthalic acid. The holes of the valence band of g-C 3 N 4 (E VB = 1.5 eV) and electrons of the conduction band of Ag 2 CO 3 (E CB = +0.5 eV) are useless charge carriers with a weak redox potential and are attracted toward each other by coulombic attraction force and removed leaving a strong built-in internal electric eld. On the contrary, the valence band holes of Ag 2 CO 3 (E VB = +2.5 eV) and electrons in the conduction band of g-C 3 N 4 (E CB = −1.3 eV) were consumed readily in the photocatalytic process. Ag 2 CO 3 positive holes with a potential of 2.55 eV generate hydroxyl radicals (E OH − /OHc = + 2.4 eV) and electrons in the CB of g-C 3 N 4 with a potential of −1.13 eV generate superoxide radicals ðE O2=O 2 ¼ À0:34Þ. The classical staggered type (II) heterojunction cannot give a true pathway for charge diffusion between oxidative Ag 2 CO 3 and reductive g-C 3 N 4 sheets. The electrons jumping from the conduction band of g-C 3 N 4 to the conduction band of Ag 2 CO 3 fail to produce superoxide radicals ðE O2=O 2 ¼ À0:34 eV Þ and the transfer of holes from the Ag 2 CO 3 valence band to the valence band of g-C 3 N 4 cannot oxidize water to produce hydroxyl radicals ðE OH À =OH ¼ þ2:4 eV Þ. On the basis of the aforementioned results, the S-scheme heterojunction is the actual mechanism for explaining the precise charge transportation in the Ag 2 CO 3 / g-C 3 N 4 heterojunction. Table 1 provides the comparative study of the decomposition of various organic dyes on the surface of Ag 2 CO 3 /g-C 3 N 4 prepared via various approaches such as hydrothermal, co-precipitation and sonochemical routes. The results recorded in the previous research studies manifest the requirement of a large proportion of Ag 2 CO 3 in the nanocomposite, which is a negative economic factor due to the large cost of silver precursors. The mechanism of charge transportation is discussed through the type (II) heterojunction or direct Z-scheme, which fail together in explaining the actual charge migration on the heterojunction surface. On the contrary, our research developed a low-cost and simple sonochemical route for developing Ag 2 CO 3 /g-C 3 N 4 with 5 wt% Ag 2 CO 3, which is efficient to destruct 95% of RhB dye via Sscheme heterojunctions.

Conclusions
In this research work, S-scheme Ag 2 CO 3 /g-C 3 N 4 heterojunctions have been generated by hybridizing different concentrations of spherical Ag 2 CO 3 nanoparticles and g-C 3 N 4 sheets sonochemically for expelling RhB dye. The experimental analysis indicates that the localized deposition of Ag 2 CO 3 on g-C 3 N 4 sheets, deep absorption of solar radiation, better electron hole-electron separation and transportation are the main parameters for optimizing the photocatalytic efficiency of the heterojunction composed of 5 wt% Ag 2 CO 3 and 95% g-C 3 N 4 sheets. The destruction of Rhodamine B dye passes through a series of intermediate organic compounds that nally decompose completely into CO 2 and H 2 O. The charge transportation via the S-scheme photocatalytic mechanism was manifested from scrubber trapping experiments and PL analysis of terephthalic acid. Superoxide radicals in addition to positive holes demonstrate a predominant contribution role in degrading Rhodamine B dye under natural sunlight of 500 W intensity.

Conflicts of interest
There are no conicts to declare.